1. Field of the Invention
This invention relates to the shaping of heat-softenable materials such as glass and plastic materials that are used as substitutes for glass in aircraft transparencies where light weight is an important factor in addition to good optical properties. Typical organic compositions include acrylic plastics, polycarbonates, polyester resins and certain polyurethanes.
Aircraft transparencies include both windows and closings for pilot cockpits. The latter are generally in the form of a complicated bubble whose cross-section at different portions along its length defines arcs of different curvature and whose curve extends transversely around more than 180 degrees of arc of a circle, ellipse or other rounded shape. In addition, the longitudinal dimension of the bubble is curved about an axis extending transverse to its length. Such shapes are needed in aircraft so that the contour of the transparencies follows the shape of the aircraft fuselage in order to make the aircraft as streamlined as possible. In addition, it is necessary that a pilot sitting in the cockpit have good visibility through the cockpit transparency so as to have a good view in all directions from the aircraft.
When glass or lightweight plastic sheets having the requisite transparency are shaped by pressing them against solid molds having the desired curvature and configuration, the major surfaces of the sheets that ultimately form the transparency are likely to become marked. When sheets are formed by sagging due to gravity to conform to the shape of an outline mold, it is difficult to maintain the shape of the glass within the outline that conforms to the outline mold. Other means of forming glass or plastic sheets while heat-softened, such as blow forming and vacuum forming, have their limitations in the complexity to which the sheets can be shaped.
During blow forming, the thickness of the glass sheet that results from such shaping is non-uniform and the degree of non-uniformity varies considerably with the size of the sheet and the degree of curvature to which the sheet is to be shaped.
At the time of the present invention, the sheet shaping art required a technique for shaping sheets of glass or transparent plastic materials to complicated shapes needed for various aircraft transparencies in such a manner that the sheets would avoid problems due to inadequate shaping and/or the marking of the major surfaces and/or the development of non-uniform thickness during the shaping operation.
Some of the recent shapes required involve the F-16 aircraft canopy and also transparencies for the F-15 aircraft. One of the complicated parts that the present invention can produce has a complicated shape that includes a semi-cylindrical shape at one end and a bend of varying radius extending well over 180 degrees of arc around the circumference of a rounded shape at the other end. The patents available in the prior art were not capable of producing such complicated bends without introducing optical deficiencies and/or causing non-uniform thickness in the workpiece.
2. Description of Patents of Interest
U.S. Pat. No. 2,123,552 to Helwig discloses the blow forming of plastic sheeting against a so-called negative mold having a three dimensional curved surface allegedly without marring the surface of the sheet undergoing shaping. The plastic sheet is blow formed against a mold having a convex shaping surface by the application of fluid pressure. When the fluid pressure exceeds a predetermined amount, which is associated with the completion of the shaping, the pressure lifts the mold. This actuates a solenoid valve circuit which stops the application of pressure so as to produce a shaped plastic sheet that does not come into contact with the mold shaping surface and which, as a consequence, has optically clear surfaces.
U.S. Pat. No. 2,218,654 to Paddock discloses a two-step method for shaping glass sheets to deep bends. The first step is an over bend where the glass, while supported on pipes between its ends and on additional pipes at its ends is heated to a glass softening temperature. The end supporting pipes are removed to permit the glass sheet to sag into a convexly elevated shape. The glass sheet is then turned upside-down and shaped additionally by gravity sagging through the application of heat while the upside-down glass sheet is supported over a gravity bending mold having a shaping surface of concave elevation.
U.S. Pat. No. 2,377,849 to Binkert and Jendrisak discloses a process of bending a stack of glass sheets by prebending them by gravity to conform to the shape of a first mold followed by shaping the glass sheets by a combination of gravity and suction to conform to a second mold of compound curvature.
U.S. Pat. No. 2,420,119 to Boehm and Ladon discloses a method of shaping sheet material by applying rollers over one surface of a sheet that is rolled to conform to the outer shape of a male mold. The rollers are not capable of providing the optical quality needed for aircraft transparencies, because they roll against the vision area of the sheet during shaping.
U.S. Pat. No. 2,474,652 to Block discloses a method that uses a hinged bending mold to form a sheet comprising composite laminated structures of metal and wood. The final shape of the contour is defined only by a clamping rail and a center hold down member. Since the center hold down member forming part of the apparatus of this patent engages the entire length of the composite to be shaped, a composite composed of a transparent material would have a line of optical defect extending across the center of the fabricated article.
U.S. Pat. No. 2,510,304 to Ames relates to centrifugally forming canopies. This technique is entirely different from the technique of the present invention and would result in optical defects when the canopy contacts the mold.
U.S. Pat. No. 2,518,896 to Jendrisak discloses a technique of bending a pair of glass sheets supported in an oblique orientation over an outline shaping mold. A pivotable frame member of complemental shape rests on the upper shaping surface of the glass sheet when the latter is heat-softened to pressure form the glass to a shape conforming to that of the upwardly facing outline shaping surface of the mold beneath the glass sheet.
U.S. Pat. No. 3,020,596 to Clapp and Jameson forms a heat softened sheet to a cylindrical shape by draping it over a male mold of convex elevation. Since gravity is the only force and depends on the weight of the plastic sheet, it is quite possible that gravity alone will be insufficient to pull the weight of the heat softened sheet to the exact configuration of the male mold. Such a technique cannot produce any complicated shape that includes a reverse sag and cannot form parts that are shaped to a contour that extends more than 180 degrees of arc around a circumference.
U.S. Pat. No. 3,574,807 to Heavener discloses a method of molding plastic sheets to a V-type bend using an angulated cavity form as a bending mold. A pair of metal bars are used to provide rigidity in the direction perpendicular to the bending axis in order to ensure that the sheet to be shaped bends about a single axis.
U.S. Pat. No. 3,607,188 to Stilley and Eilenfeld covers a method and apparatus for shaping a glass sheet to a hemispherical shape by first applying a plunger to the upper surface of the heated glass sheet while the latter is spaced above a mold to accelerate the initial sagging of the sheet. The downward movement of the plunger is limited so that the sheet sags away from the plunger and toward the mold. This patent minimizes surface damage by avoiding simultaneous engagement of both major surfaces of the sheet to be shaped in its viewing area. However, this patented apparatus does not teach one skilled in the art how to bend a sheet to a shape having a cross-section of arcuate shape extending more than 180 degrees.
U.S. Pat. No. 4,002,417 to Vecchiotti and Goldberg discloses apparatus for folding a sheet of plastic material along a fold line using a mechanical linkage mechanism. This patent is incapable of producing complicated shapes presently required in aircraft transparencies.
U.S. patent application Ser. No. 18,964 of Comperatore, Wise, and Black, filed Mar. 9, 1979, now U.S. Pat. No. 4,233,050 covers shaping glass sheets to complicated shape by first preliminarily shaping said sheets to a prebent shape within a thin flexible blanket of fiber glass cloth and converting the prebent shape to a final complicated shape during final shaping by blow forming. This application also covers a technique of cradling glass sheets in a flexible fiber glass blanket that is wrapped over the margin of the glass sheet to be shaped by blow forming. The margin of the blanket is reversely wrapped over a frame-like ring that prevents the marginal edge from curling and maintains the cradle in tension against the lower major surface of the glass sheet during final shaping by blow forming.
U.S. patent application Ser. No. 25,137 of Comperatore, Black and Wise, filed Mar. 29, 1979, now U.S. Pat. No. 4,229,201 covers apparatus for shaping a glass sheet to a complicated shape including a pair of sharply bent areas extending across its transverse dimension and a substantially spherically sagged portion intermediate the areas of sharp bending by a combination of gravity sag bending and localized intense line heating.